As we have seen in the earlier verses, the Jewish leaders have been trying to convince the new converts to return to the old Jewish traditions. Paul is making the point here that they need to consider the hardships and severity of the law before they turn back to it. He is using the birth of Issac and Ishmael to demonstrate the difference in natural and spiritual things. Those that return to worldly things (the natural Jerusalem) are still in slavery to those things. But since Issac was born to fulfill a divine promise, there is now a choice that we can make. We can follow the fleshly lineage and remain slaves or we can follow the divine lineage, accept Jesus and become citizens of the Jerusalem which is above.
20 But our citizenship is in heaven. And we eagerly await a Savior from there, the Lord Jesus Christ, 21 who, by the power that enables him to bring everything under his control, will transform our lowly bodies so that they will be like his glorious body.–Phil. 3:20 (NIV)
But one day, the New Jerusalem will come down to earth and all those that have chosen the divine lineage (Jesus) will be citizens of the New Jerusalem.
I saw the Holy City, the new Jerusalem, coming down out of heaven from God, prepared as a bride beautifully dressed for her husband—Rev.21:2 (NIV)